EarthquakesEarthquakes

SlinkyBeaker, Wet Sand, WeightCardboard Fault modelsChewing GumWood meter stickpencil

What is an earthquake?What is an earthquake?

An earthquake is the vibration of Earth An earthquake is the vibration of Earth produced by the rapid release of energyproduced by the rapid release of energy

Energy radiates in all directions from its Energy radiates in all directions from its source, the source, the focusfocus

Energy moves like wavesEnergy moves like waves Seismographs record the eventSeismographs record the event

Anatomy of EarthquakesAnatomy of Earthquakes

Earthquakes and faultsEarthquakes and faults Earthquakes are associated with faultsEarthquakes are associated with faults Motion along faults can be explained by plate Motion along faults can be explained by plate

tectonicstectonics

Causes of earthquakesCauses of earthquakes

Sudden release of accumulated Sudden release of accumulated strain energystrain energy

Creation of new faults by Creation of new faults by rupturing rocksrupturing rocks

Shifting of rocks at preexisting Shifting of rocks at preexisting faultsfaults

Elastic rebound 1Elastic rebound 1

Mechanism for EarthquakesMechanism for Earthquakes– Rocks on sides of fault are deformed by Rocks on sides of fault are deformed by

tectonic forces tectonic forces

– Rocks bend and store elastic energyRocks bend and store elastic energy

– Frictional resistance holding the rocks Frictional resistance holding the rocks

together is overcome by tectonic forces together is overcome by tectonic forces

Elastic rebound 2Elastic rebound 2

Earthquake mechanism Earthquake mechanism

– Slip starts at the weakest point (the focus) Slip starts at the weakest point (the focus) – Earthquakes occur as the deformed rock Earthquakes occur as the deformed rock

“springs back” to its original shape (“springs back” to its original shape (elastic elastic reboundrebound))

– The motion moves neighboring rocksThe motion moves neighboring rocks– And so on.And so on.– DEMO – elastic rebound w/ meter stickDEMO – elastic rebound w/ meter stick

) ) ) ) ) ) )( ( ( ( ( ( (

AftershocksAftershocks

The change in stress that follows The change in stress that follows a mainshock creates smaller a mainshock creates smaller earthquakes called earthquakes called aftershocksaftershocks

The aftershocksThe aftershocks““illuminate” the faultilluminate” the fault that ruptured in the mainshockthat ruptured in the mainshock

Red dots show location ofaftershocks formed by 3earthquakes in Missouriand Tennessee in 1811/1812

RelationRelationship ship BetweeBetween Stress n Stress and and StrainStrain

Strain can be a change in shape (a deformation) due to an applied stress

Relationship Relationship Between Between Stress and Stress and Strain at low Strain at low Temps and Temps and Pressure or Pressure or Sudden Sudden StressStress

Relationship Relationship Between Between Stress and Stress and Strain under Strain under high Temps high Temps or Pressure or Pressure

Strike and DipStrike and Dip

Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface

Strike is long line, dip is short lineNote the angle of dip given 45o

Dip-Slip Dip-Slip FaultsFaults

Horizontal Movement Along Horizontal Movement Along Strike-Slip FaultStrike-Slip Fault

Normal Fault Quake - NevadaReverse Fault Quake - Japan

Strike Slip Fault Quake - California

DEMO – Types of faults

San Andreas: An active San Andreas: An active earthquake zoneearthquake zone

San Andreas is the most studied fault system in San Andreas is the most studied fault system in the worldthe world

Displacement occurs along discrete segments 100 Displacement occurs along discrete segments 100 to 200 kilometers longto 200 kilometers long

Most segments slip every 100-200 years Most segments slip every 100-200 years producing large earthquakesproducing large earthquakes

Some portions exhibit slow, gradual Some portions exhibit slow, gradual displacement known as fault creepdisplacement known as fault creep

Fence offset by the 1906 Fence offset by the 1906 San Francisco earthquakeSan Francisco earthquake

Fires caused by 1906 San Francisco Earthquake

Gas mains break, fires shaken out of furnaces.Water mains break, cannot fight fires. Debris in streets, Fire department cannot reach fires.

Landscape Shifting, Wallace Landscape Shifting, Wallace CreekCreek

LiquefactionLiquefaction

Demo: Liquifaction

Analogy: Feet at the shore

SeismologySeismology

SeismometersSeismometers - instruments that - instruments that record seismic wavesrecord seismic waves

Records the movement of Records the movement of Earth in relation to a stationary Earth in relation to a stationary mass on a rotating drum or mass on a rotating drum or magnetic tapemagnetic tape

A seismograph designed to A seismograph designed to record vertical ground record vertical ground

motionmotionThe heavy mass doesn’t move much

The drum moves

Lateral Movement DetectorLateral Movement Detector

In reality, copper wire coils move around magnets, generating current which is recorded.

Types of seismic wavesTypes of seismic waves

Surface wavesSurface waves

–Complex motion, great destructionComplex motion, great destruction

–High amplitudeHigh amplitude and low velocity and low velocity

–Longest periods (interval between Longest periods (interval between crests)crests)

–Termed long, or L wavesTermed long, or L waves

Two Types of Surface Two Types of Surface WavesWaves

Types of seismic waves (continued)Types of seismic waves (continued) Body wavesBody waves

– Travel through Earth’s interiorTravel through Earth’s interior – Two types based on mode of travelTwo types based on mode of travel – Primary (P) wavesPrimary (P) waves

Push-pull motionPush-pull motion Travel thru solids, liquids & gasesTravel thru solids, liquids & gases

– Secondary (S) wavesSecondary (S) waves Moves at right angles to their Moves at right angles to their

direction of traveldirection of travel Travels only through solidsTravels only through solids

Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L

P and S waves

Demo: P and S waves

Locating the source of Locating the source of earthquakesearthquakes

FocusFocus - the place within Earth where - the place within Earth where earthquake waves originateearthquake waves originate

EpicenterEpicenter – location on the surface directly – location on the surface directly above the focusabove the focus

Epicenter is located using the difference in Epicenter is located using the difference in velocities of P and S wavesvelocities of P and S waves

Earthquake focus and Earthquake focus and epicenterepicenter

Note how much bigger the surface waves are

Graph to find distance to Graph to find distance to epicenter epicenter

Locating the epicenter of an earthquakeLocating the epicenter of an earthquake

Three seismographs needed to locate an Three seismographs needed to locate an epicenterepicenter

Each station determines the time interval Each station determines the time interval between the arrival of the first P wave and between the arrival of the first P wave and the first S wave at their locationthe first S wave at their location

A travel-time graph then determines each A travel-time graph then determines each station’s distance to the epicenterstation’s distance to the epicenter

Locating Earthquake Epicenter Locating Earthquake Epicenter

Locating the epicenter of an earthquakeLocating the epicenter of an earthquake

A circle with radius equal to distance to A circle with radius equal to distance to the epicenter is drawn around each the epicenter is drawn around each stationstation

The point where all three circles The point where all three circles intersect is the earthquake epicenterintersect is the earthquake epicenter

Epicenter located using three Epicenter located using three seismographsseismographs

Earthquake BeltsEarthquake Belts

95% of energy released by earthquakes 95% of energy released by earthquakes originates in narrow zones that wind originates in narrow zones that wind around the Eartharound the Earth

These zones mark of edges of These zones mark of edges of tectonic platestectonic plates

Locations of earthquakesLocations of earthquakesfrom 1980 to 1990from 1980 to 1990

Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates

Depths of EarthquakesDepths of Earthquakes

Earthquakes originate at depths ranging from 5 to , Earthquakes originate at depths ranging from 5 to , rarely, nearly 700 kilometersrarely, nearly 700 kilometers

WHY 700. Deeper, rock deforms plasticallyWHY 700. Deeper, rock deforms plastically

Cannot spring back elastically, no earthquakeCannot spring back elastically, no earthquake

DEMO: Plastic DeformationDEMO: Plastic Deformation

Definite patterns existDefinite patterns exist in depth in depth

– Shallow focus 20 km faults between MOR Shallow focus 20 km faults between MOR

– Large Transform (San Andreas) fault 80 kmLarge Transform (San Andreas) fault 80 km

– Deep earthquakes occur in Pacific landward of oceanic Deep earthquakes occur in Pacific landward of oceanic trenches 80 km down to about 600 kmtrenches 80 km down to about 600 km

Associated with subduction zonesAssociated with subduction zones

Earthquake Depth and Plate Tectonic Earthquake Depth and Plate Tectonic SettingSetting

Subduction Zones discovered by Benioff

Earthquake in subduction Earthquake in subduction zones zones

Earthquakes at Divergent Earthquakes at Divergent Boundaries - IcelandBoundaries - Iceland

Crust pulling apart – normal faults

Measuring the size of Measuring the size of earthquakesearthquakes

Two measurements describe the size of an Two measurements describe the size of an earthquakeearthquake

IntensityIntensity – a measure of earthquake shaking – a measure of earthquake shaking at a given location based on amount of at a given location based on amount of damagedamage

MagnitudeMagnitude – estimates the amount of energy – estimates the amount of energy released by the earthquakereleased by the earthquake

Intensity scalesIntensity scales

Modified Mercalli Intensity ScaleModified Mercalli Intensity Scale was was developed using California buildings as its developed using California buildings as its standardstandard

Drawback is that destruction may not be Drawback is that destruction may not be true measure of earthquakes actual severitytrue measure of earthquakes actual severity

Earthquake destructionEarthquake destructionAmount of structural damage depends Amount of structural damage depends

onon Intensity and duration of vibrationsIntensity and duration of vibrations Nature of the material upon which the Nature of the material upon which the

structure rests (hard rock good, soft structure rests (hard rock good, soft bad)bad)

Design of the structureDesign of the structure

Magnitude scalesMagnitude scales

Richter magnitudeRichter magnitude - concept introduced by - concept introduced by Charles Richter in 1935Charles Richter in 1935

Richter scaleRichter scale

–Based on amplitude of largest seismic Based on amplitude of largest seismic wave recordedwave recorded

–LOGLOG1010 SCALE SCALE

Each unit of Richter magnitude Each unit of Richter magnitude corresponds to 10X increase in wave corresponds to 10X increase in wave amplitude and 32X increase in Energyamplitude and 32X increase in Energy

Magnitude scalesMagnitude scales

Moment magnitudeMoment magnitude was developed because was developed because Richter magnitude does not closely estimate Richter magnitude does not closely estimate the size of very large earthquakesthe size of very large earthquakes

–Derived from the amount of displacement Derived from the amount of displacement that occurs along a fault and the area of that occurs along a fault and the area of the fault that slipsthe fault that slips

TsunamisTsunamis, or seismic sea waves, or seismic sea waves

Destructive waves called “tidal waves”Destructive waves called “tidal waves” Result from “push” of underwater fault Result from “push” of underwater fault

or undersea landslideor undersea landslideIn open ocean height is > 1 meterIn open ocean height is > 1 meter In shallow coast water wave can be > 30 In shallow coast water wave can be > 30

metersmeters Very destructiveVery destructive

Formation of a tsunamiFormation of a tsunami

Tsunamis are actually huge, extending from the fault on the sea floor up to the surface, but they don’t stick up more than a meter or so in the deep ocean. However, when they reach shallow water they must rear up and slow down. Discussion: Kinetic vs. potential energy

Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere

Tsunami 1960, Tsunami 1960, Hilo HawaiiHilo Hawaii

Tsunami Model, Japan Tsunami Model, Japan EarthquakeEarthquake

Tsunami Tsunami Model, Model, Alaska Alaska QuakeQuake

Earthquake predictionEarthquake prediction

Long-range forecasts Long-range forecasts Calculates probability of a certain Calculates probability of a certain

magnitude earthquake occurring over a magnitude earthquake occurring over a given time periodgiven time period

Short-range predictionsShort-range predictions Ongoing research, presently not much Ongoing research, presently not much

successsuccess

Long Term Predictions

Seismic Gaps

Long Term PredictionsLong Term Predictions Strain Energy - accumulates Strain Energy - accumulates

uniformly - release irregularlyuniformly - release irregularly Some locked by friction “Seismic Some locked by friction “Seismic

gaps”gaps”– Prime candidates for major Prime candidates for major

earthquakeearthquake Some release energy Some release energy

continuously- creepcontinuously- creep– No major earthquakes thereNo major earthquakes there

Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE

Seismic Gap along HimalayasSeismic Gap along Himalayas

2005

Can earthquakes be Can earthquakes be predicted?predicted?

Short Term, Not very wellShort Term, Not very well Short-range predictionsShort-range predictions

Goal is to provide a warning of the location Goal is to provide a warning of the location and magnitude of a large earthquake within and magnitude of a large earthquake within a narrow time framea narrow time frame

Research has concentrated on monitoring Research has concentrated on monitoring possible precursors – phenomena that possible precursors – phenomena that precede a forthcoming earthquake such as precede a forthcoming earthquake such as measuringmeasuring uplift, subsidence, and strain in uplift, subsidence, and strain in the rocksthe rocks

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Dilatancy of Highly Stressed Dilatancy of Highly Stressed RocksRocks

Short-Term Earthquake Prediction

Investigating Earth’s InteriorInvestigating Earth’s Interior Seismology helps us understand Earth’s Seismology helps us understand Earth’s

Interior Structure. We use:Interior Structure. We use: Speed changes in different materials Speed changes in different materials

due changes rigidity, density, elasticity due changes rigidity, density, elasticity Reflections from layers with different Reflections from layers with different

propertiesproperties Attenuation of Shear Waves in fluidsAttenuation of Shear Waves in fluids Direction changes (Refraction)Direction changes (Refraction)

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Three Major Components of Three Major Components of EarthEarth

Three Major Components of Three Major Components of EarthEarth

!

Seismic-wave velocities are faster in the upper Seismic-wave velocities are faster in the upper mantlemantle

Waves that travel via mantle arrive sooner at far destinations

Velocity increases w depth, waves bend back to surface.

Mohorovičić discontinuity

Wave VelocitiesWave Velocities

Upper Mantle Fast

Asthenosphere Slow

Lower Mantle Fast

http://pubs.usgs.gov/gip/interior/

Mineralogy of Earth’s LayersMineralogy of Earth’s Layers

The P-Wave Shadow ZoneThe P-Wave Shadow Zone

Behavior of waves through center reveal Earth’s Interior

P-waves through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole

HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid.Inge Lehmann

http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html

The S-Wave Shadow ZoneThe S-Wave Shadow Zone

Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source.

http://en.wikipedia.org/wiki/Richard_Dixon_Oldham

End of End of EarthquakesEarthquakes